That's correct - there's no REST calling of the Trading API, just HTTP POSTs. eBay removed that functionality back in around 2008 or so.

(Why did they remove REST from Trading? I'm guessing it's because the Shopping API was launched and stable by then, with REST capabilities, and it might've simplified their maintenance/management of the Trading API to not have to handle incoming REST calls. ...Just my $0.02 there.)